Transparent personal cleansing bars are well known in the art. They are prepared by dissolving soap in a combination of water soluble solvents (normally urea and/or polyhydric alcohols) with the addition of surfactants to improve the cleansing and lathering characteristics of the product. Water is also part of the composition and is incorporated into the bars by the direct addition of water or added from the surfactants or soap that form part of the cleansing bars.
A common problem with transparent personal cleansing bars is that they have a melting point of about 50.degree. C., which is too close to the temperatures that are reached during shipping or storage of the products. When shipped or stored at temperatures approaching 50.degree. C., the solid personal cleansing bars either melt or become sufficiently soft to deform. Another common problem of the personal cleansing bars is that they lose more weight than regular, opaque cleansing bars due to their relatively higher content of water, and the presence of volatile solvents, such as monohydric alcohols, e.g., ethanol.
U.S. Pat. No. 5,041,234 to Instone, et al. discloses a transparent personal cleansing bar which contains greater than 40 percent by weight of soap (high soap bar). Like other "high soap" transparent bars, the bars disclosed by Instone et al. have good lather, low smear, and good bar hardness. Such high soap level transparent bars, however, are rather harsh to the skin. Another drawback to such bars is that their processing generally require the use of at least about 5% by weight volatile, short chain monohydric alcohols, or require special milling to obtain transparency.
Transparent bars which contain lower levels of soap are also known in the art. "Lower soap" transparent bars contain less than 40 percent by weight of soap. U.S. Pat. No. 5,002,685 to Chambers et al. discloses a transparent bar made with 25% to 34% by weight soap, 5% to 15% by weight monohydric alcohol, 15% to 30% by weight sugar and/or cyclic polyol, and 15% to 30% by weight water. Unfortunately, transparent bars which require the use of at least about 5% by weight monohydric alcohols are prone to excessive weight loss due to the volatile nature of most monohydric alcohols. Such transparent bars are also more expensive to prepare and require special equipment designed to accommodate the explosion hazard associated with most monohydric alcohols.
Zyngier et al. U.S. Pat. No. 5,703,025 discloses a monohydric alcohol-free process for making pour molded transparent and translucent cleansing bars that have good hardness, are mild to the skin, low smearing and good lathering. These bars, however, require at least 14% by weight water, particularly 14% to 32% by weight water, and preferably more than 20% by weight water, so that the bars melt and deform at a relatively low temperature, e.g., about 50.degree. C., and readily lose weight via water evaporation at room temperature and pressure. As stated in the '025 patent, "The water level within the personal cleansing bars prepared by the process of the present invention is critical to obtain a transparent bar having desirable hardness characteristics. When the water is less than about 14 parts by weight of the bar, the bar may not be transparent."
A transparent/translucent cleansing bar containing 20% by weight water loses about 17% of its original weight when exposed to room temperature for three weeks, whereas a bar containing only 10% by weight water only loses about 7.5% of its original weight under the same, standard temperature (room temperature) and pressure conditions.